One important step in the fabrication of multilayered printed wiring boards is the removal of drill smear. Drill smear is found in holes drilled through multi-layer printed wiring boards. Drill smear is formed by dielectric material that is melted, or smeared, into an electrical connection hole by the heat generated by a drill bit as the hole is being bored. The smear covers the abutting edges of conductors on the individual boards. This makes it difficult to metalize the hole to connect together the conductors that abut the hole. Thus, it is necessary to remove the smear, a process called "desmearing", prior to the later metalizing process.
In the past, concentrated sulfuric acid has been used as a desmearing agent. However, it has not always proved to be efficient. Its viscosity and strong accidity make it difficult to work with. With large holes, (0.043 in. in radius or greater) if the acid is not removed quickly, it will etch holes of uneven size. This makes it difficult to properly metalize the holes. With small holes (less than 0.043 in. in radius), the viscous nature of the sulfuric acid prevents it from flowing into the holes. This reduces the thoroughness of the desmearing process, with resultant problems in the subsequent electroplating process.
Recently, it has been found that potassium permanganate works well in a desmearing solution.It removes all traces of smear on printed wire boards, yet, it is not as corrosive as sulfuric acid. As a result, it is becoming a preferred desmearing agent.
However, the chemical ractions involving the use of potassium permanganate are highly oxidizing, and produce a precipitate of manganese dioxide. The precipitate can agglomerate and thereby plug up small holes, defeating the purpose of the desmearing. Also, large amounts of precipitate settle in the bottom of the tank and form a semi-solid sludge. This makes it impossible to place printed wiring board near the bottom of the tank for they quickly become covered with a layer of precipitate. These problems have become more serious with the trend to greater component density on the boars and attendant reduction in hole sizes. Reducing the bath life of each batch of potassium permanganate has not appreciably reduced this problem.
There is also a problem of disposal of the desmearing bath when potassium permanganate has been used. It is difficult and expensive to clean out the sludge from the desmearing tank. Also, the sludge contains free liquid that is a hazardous waste, and thus must be disposed of in a relatively costly manner.
One suggested way to reduce the precipitate problems was to batch filter each desmearing bath containing potassium permanganate. Each bath was first allowed to cool to room temperature (the desmearing solution must be heated prior to sue heat). A slurry of solution and precipitate was then pumped through a filter. However, the precipitate immediately formed a paste-lke coating on the filter, making it impossible to filter additional solution.
A need therefore exists for a method to remove manganese dioxide precipitate from a potassium permanganate desmearing solution. This method should reduce the amount of precipitate in the solution to a low level to minimize precipitate clogging of holes in the printed wire board. Reducing the amount of precipitate will also extend the bath life of each batch of potassium permanganate and make it easier to clean the desmearing tank between baths.